1. Field of the Invention
Embodiments of the invention relate to methods and devices for use in evaluating ophthalmic formula effects on the tear film and to use such information to diagnose ophthalmic formula treatment of ocular disease conditions such as dry eye or post-surgical ophthalmic formula treatment and diagnosis.
2. Description of the Related Art
Dry eye syndrome is a prevalent condition among both men and women for which there is no cure, although symptoms may be relieved with proper diagnosis and treatment. The condition affects more than 3.2 million American women middle-aged and older alone (Schaumberg D A, Sullivan D A, Buring J E, Dana M R. Prevalence of dry eye syndrome among US women. Am J Ophthalmol 2003 August; 136(2):318-26). Most dry eye patients are prescribed artificial tears to treat their dry eye conditions. Contact lens wearers, computer users, patients who live and/or work in dry environments, and patients with autoimmune disease are all particularly susceptible to developing dry eye.
Individuals with moderate to severe dry eye are unsuitable for contact lens wear and must wear eyeglasses or undergo refractive surgery for their vision correction needs. LASIK refractive surgery induces some degree of dry eye in virtually all patients for a period of time, sometimes six months or more. Cataract surgery also induces some degree of dry eye in a substantial number of patients for a period of time. It may be desirable to prescribe artificial tears for LASIK and cataract surgery patients to treat their dry eye condition.
Current methods for diagnosing dry eye in the absence of contact lens wear utilize methods such as symptom assessment, fluorescein staining, tear film break-up time (TBUT), non-invasive tear film break-up time (NITBUT), Schirmer test, Phenol red thread test, rose bengal or lissamine green staining, conjunctival hyperemia, tear film osmolarity, tear lactoferrin, impression cytology, brush cytology, “tear assessment”, blink frequency and maximum interblink interval. For various reasons, all of these methods are imperfect and lacking in precision.
Symptom assessment is most often used for dry eye diagnosis, in the absence or presence of contact lens wear. It is a subjective and qualitative assessment, but was nonetheless used in 82.8% of all diagnoses of dry eye in a recent study (Nichols K K, Nichols J J, Zadnik K. Frequency of dry eye diagnostic test procedures used in various modes of ophthalmic practice. Cornea 2000 July; 19(4):477-82).
Fluorescein staining of the cornea is frequently used for dry eye diagnosis, being used in 55.5% of all diagnoses in a recent study (IBID). This is a semi-quantitative assessment which typically consists of dividing the cornea into 5 sections and assessing staining intensity on a 5-point scale. Most fluorescein staining methods also include an assessment of % surface area of staining within each corneal section after the instillation of fluorescein dye into the eye.
Tear film break-up time (TBUT) is another test which is relatively frequently used for dry eye diagnosis, with or without contact lens wear. It was used in 40.7% of all diagnoses of dry eye in a recent study (IBID). The tear film is a continuous film covering the eye. However, it is unstable and breaks up after a short period of time. In patients with dry eyes, the tear film breaks up faster. TBUT measurements are facilitated with the use of fluorescein instilled into the eye with the use of a fluorescein strip. However, the instillation of fluorescein often stimulates reflex tearing, obviating the measurement of TBUT. Also, the presence of fluorescein in the tear film changes the properties of tears, which means that the measurements may not be truly physiological. TBUT measurements are also not precise.
The non-invasive tear film break-up time (NITBUT) method was developed to overcome the limitations of the TBUT method. With the NITBUT method, the eye is observed with a keratometer, hand-held keratoscope or tearscope. The reflections of keratometer mires are observed and the time is measured for a mire to break up following a blink. There is nonetheless considerable variation of NITBUT measurements. Furthermore, tear breakup time is abnormal in many different dry eye states and thus cannot easily differentiate between dry eye types.
The Schirmer test measures the amount of aqueous tears that can be produced by the eye in 5 minutes. If too little aqueous tears are produced, this is indicative of an aqueous deficient dry eye. If enough tears are produced, but symptoms of dry eye exist, this is indicative of an evaporative dry eye for example due to a lipid deficiency, blepharitis or Rosacea. In the Schirmer test, a 35 mm×5 mm filter paper strip is placed into the lower cul-de-sac of the eye and allowed to wet over its length over 5 minutes. Schirmer tests are performed without and with prior application of an anesthetic eyedrop. When an anesthetic eyedrop is not used, the test is considered to measure basal+reflex tear secretion. When an anesthetic eyedrop is used, the test is considered to measure only basal tear secretion. Most clinicians regard the Schirmer test as unduly invasive and of little value for the diagnosis of mild to moderate dry eyes. The test cannot properly diagnose lipid deficient dry eyes. The Schirmer test cannot properly diagnose dry eye states wherein sufficient or even excess aqueous tears are produced. The test, with or without prior use of an anesthetic eyedrop, is also considered to lack precision and accuracy. There is considerable overlap in Schirmer test values between patients with Keratoconjunctivitis sicca (dry eye) and normals.
The Phenol red thread test was developed as a less invasive method than the Schirmer test. It involves the use of a cotton thread impregnated with phenol red dye. The dye changes color from yellow to red when contacted by aqueous tears. The crimped end of a 70 mm long thread is placed in the conjunctival formix. After 15 seconds, the length of the color change in the thread is measured in millimeters. This test is also nonetheless still invasive and lacks sufficient precision and utility for mild to moderate dry eye diagnosis.
Rose Bengal staining is infrequently used as a dry eye diagnostic. The test involves the instillation of rose bengal dye into the eye and then performing a visual assessment of conjunctival staining. Rose Bengal staining is dependent upon secondary changes in the ocular surface caused by the primary changes due to dry eye and is a good parameter for aqueous tear deficiency only in the absence of other ocular surface diseases.
Conjunctival hyperemia is a subjective assessment of ocular redness. Since redness occurs in ocular conditions other than dry eye (e.g., during infection), this test is unsuitable as an independent diagnostic for dry eye.
Osmolarity, lactoferrin, impression cytology and brush cytology diagnostic methods all involve substantial chemical laboratory work and are thus not suited for general clinical use. Osmolarity cannot independently diagnose dry eye conditions wherein sufficient or even excess aqueous tears are produced.
Tear assessment includes an assessment of total aqueous tear fluid volume via an assessment of inferior tear meniscus height, inferior tear meniscus radius of curvature or meniscus area. Since this method does not evaluate the tear film lipid layer, it is not accurately diagnostic for tear lipid deficiencies, which account for 60% or more of dry eye cases. Tear assessment also includes an evaluation of the tear film lipid layer using a “tearscope”. Tearscope-based diagnoses exclude an assessment of the aqueous fluid volume and thus are limited. Additionally, the Keeler tearscope allows only a semi-quantitative analysis of the tear lipid layer, since a spectrum-color analysis of its light source has not been conducted, allowing a correlation between observed colors and thicknesses. Also, colors are still subjectively evaluated.
Blink frequency and maximum interblink interval (IBImax) have been determined to correlate to dry eye status. However, both blink frequency and maximum interblink interval measurements have not been routinely used to diagnose dry eye due to the inherent complexity of their measurement, involving video recording and video frame analysis.
Several methods have been employed for measuring the ocular retention times of ophthalmic formulations such as artificial tears used to treat dry eye. Sodium fluorescein has been added to an ophthalmic formulation and the fluorescence signal has been monitored with time using a slit lamp fluorophotometer. This method suffers from at least two problems: first, the fluorescein washes out of the eye at a rate different from that of the formulation components of interest and secondly it diffuses into the ocular tissue. The latter creates a source of error in formulation retention time measurements as it is difficult to distinguish between fluorescence of the thin film from fluorescence from the tissue.
Other methods for measuring the retention times of ophthalmic formulations in the eye include gamma scintigraphy. However, these methods involve the use of radioisotopes and therefore necessitate expensive equipment and a laboratory suited for the handling of isotopes. Also, the radioactive compounds typically have low molecular weights so they too may freely diffuse out of the viscous vehicle and into ocular tissue or be deposited on the lid margins that will result in erroneous retention measurements.
U.S. Pat. No. 5,634,458 discloses a method for determining precorneal retention time of ophthalmic formulations employing a high-molecular weight fluorescein molecule, to avoid tissue uptake of fluorescein. While this method tracks the fluorescence of the high molecular weight fluorescein to obtain a more reliable retention time, it does not measure tear film aqueous or aqueous+lipid layer thickness.
In the context of conducting research on the layers of the tear film, three general methods for measuring tear film layer thickness using optical interference have been developed, corresponding to varying one of three parameters, wavelength of light, angle of incident light and layer thickness, while keeping the other two parameters constant. These optical interferometry methods produce varying light reflection intensity profiles that have been called wavelength-dependent fringes, angle-dependent fringes and thickness-dependent fringes. Thickness-dependent fringes form the basis of the Keeler and Kowa DR-1 instruments. Wavelength-dependent fringes arise from the illumination of the tear film with a measurement beam of light of varying wavelength that intersects with a surface area of the tear film at a constant normal or near-normal angle of incidence. Provided that the tear film has an index of refraction, n, intermediate between that of the surrounding materials, e.g., air on one side and the cornea or a contact lens on the other side, and also that the refractive indices of the adjacent layers or materials are sufficiently different from one another, then the incident light wave will reflect from each boundary between layers or materials of differing refractive index. Multiple reflections will be produced, which will give rise to oscillations in the intensity of the total reflected light as a function of wavelength according to the constructive and destructive interference of the multiple reflected waves, the latter which is dependent upon the relationship between the tear film thickness, d, and the wavelength of light, λ. Maxima (peaks or fringes) in the reflectance spectrum represents the wavelengths at which constructive interference occurs between light waves reflecting at the front and back surfaces of a thin film and the minima (valleys) represent the wavelengths at which destructive interference occurs between light waves reflecting at the front and back surfaces of a thin film.
In recent years, wavelength-dependent optical interferometers have been developed for in-vivo aqueous tear film and contact lens thickness analysis by King-Smith et al., as disclosed in Fogt N and King-Smith P, Interferometric measurement of tear film thickness by use of spectral oscillations, J. Opt. Soc. Am. A/Vol. 15, No. 1/January 1998: 268-275; King-Smith P et al., The Thickness of the Human Precorneal Tear Film: Evidence from Reflection Spectra, IOVS, October 2000, Vol. 41, No. 11: 3348-3359 and Nichols J and King-Smith P, Thickness of the Pre- and Post-Contact Lens Tear Film Measured In Vivo by Interferometry, IOVS, January 2003, Vol. 44, No. 1: 68-77.
The instruments described in the aforementioned publications are of similar design and are capable of measuring the thickness of the pre-corneal or pre-lens tear film aqueous+lipid layer thickness, post-lens tear film aqueous thickness among contact lens wearers, contact lens thickness and corneal epithelial thickness. The instruments can also measure the thinning or thickening rates of the various tear film layers during normal blinking and between blinks or over time. The instruments have a high degree of quantitative accuracy and precision. However, it is reported in the January 2003 IOVS reference that the interferometer in that reference, the best of the three systems in the aforementioned three references, cannot measure mean thicknesses of less than 1 micron, meaning it cannot measure the tear lipid layer. Tear lipid layer thickness needs to be measured separately in order to determine aqueous-only layer thickness, as the light reflections from the ocular surface arise separately from the combined aqueous+lipid layer and the lipid layer alone. Wavelength-dependent fringes cannot be observed from the aqueous layer only. Lipid layer thickness would have to be measured and subtracted from the combined aqueous+lipid thickness to derive aqueous-only layer thickness. Thus, the aforementioned teaching and instruments measure thicknesses of combined aqueous+lipid layers. Since lipid layer thickness is typically only 2% of the thickness of the aqueous layer (e.g., 60 nm vs. 3000 nm), this only limits the lipid layer diagnostic capability of these instruments. The first interferometer described in the Fogt et al. 1998 reference uses a wavelength range of 369-810 nm. The second two interferometers, described in the IOVS, October 2000 reference and the January 2003 IOVS reference, utilize a wavelength range of 562-1030 nm. The instruments in the aforementioned three references are limited to measuring thickness at a single spot on the eye, approximately 300 microns round, 33×350 microns rectangular or 33×35 microns rectangular in the above three references, respectively, all at the central corneal apex. All of the aforementioned interferometers are capable of kinetic measurements of total tear film layer thickness, to produce thinning or thickening rates as well as measurements of the changes in tear film thickness over time.
Despite the optical interferometer instrument capabilities disclosed in the prior art, the effects of an ophthalmic formula topically applied directly onto the ocular surface on aqueous or aqueous+lipid or lipid tear film thickness have not been fully determined. None of the three aforementioned interferometry publications discloses measurements of tear film thickness over time following the application of an ophthalmic formula directly onto the ocular surface.
Optical coherence tomography (OCT) has most recently been used to measure changes in total tear film thickness (e.g., aqueous+lipid layer thickness) following instillation of artificial tears. 12 mm×2 mm scans of the tear film and cornea were taken at 1310±60 nm at baseline and after instillation of 35 μL of artificial tears (Refresh Liquigel™, Allergan, Irvine, Calif.). Measurements were taken at 5, 20, 40 and 60 minutes after instillation. The authors tested 40 eyes in 20 subjects and found tear film thickening in all subjects, lasting about 60 minutes. Direct measurements of the tear film were not possible, thus total tear film thickness was calculated from the subtraction of the total tear film+cornea thickness at baseline from that after instillation of the artificial tears. OCT instrument repeatability for corneal thickness was reported to be 1.5 μm. Instrument optical error was 3.7 μm, which was larger than the thickness of the normal tear film itself. Thus, this method and instrument also suffers from limited observation capabilities. Observation and measurement of changes in the aqueous or aqueous+lipid or lipid tear film layers from baseline following ophthalmic formula application are important as these allow one to measure important changes in the tear film which likely correlate to ocular surface health status, subjective comfort, optimization of ophthalmic dosage forms and drug delivery.
Given the above limitations of prior art methods for evaluation of the tear film, either alone or before and following ophthalmic formula application, it would be advantageous to have new methods which do not have some or all of the aforementioned limitations.